Antimicrobial activity of colloidal silver is well known since the 19th century. Silver is generally a safe and effective antimicrobial metal. Silver has been studied for antibacterial purposes in the form of powder, metal-substituted zeolite, metal-plated non-woven fabric, and crosslinked compound. The two main forms of silver are ionic form (Ag+) and the metallic form (Ag0) and their mechanism of action is still under debate. Antibacterial cloth containing metallic particles (particularly copper, silver, and zinc in the form of zeolite) is known in the field for a long time. Many methods for incorporating the metal ions directly into a substrate material have been proposed. However, in the methods in which the metals are used directly, the incorporation of metals leads to very expensive products, with heavy weights as they are necessarily used in large amounts.
There are also methods that use polymeric substance to hold the metallic ions. For example, the method of binding or adding fine wires or powder of the metals themselves to a polymer and the method of incorporating compounds of the metals into a polymer. However, the products obtained by these methods show poor durability of antibacterial performance and can be utilized only for restricted purposes because the metal ions are merely contained in or attached to the polymer and, accordingly, they easily fall away from the polymer while being used.
Japanese Patent No. 3-136649 discloses an antibacterial cloth that has been prepared using AgNO3. The Ag+ ions in AgNO3 are crosslinked with polyacrylonitrile. Such an antibacterial cloth demonstrates anti-bacterial activity against six bacterial strains including Streptococcus and Staphylococcus. 
Japanese Patent No. 54-151669 discloses a fiber treated with a solution containing a compound of copper and silver. The solution is evenly distributed on the fiber, which is used as an anti-bacterial lining inside boots, shoes, and pants.
U.S. Pat. No. 4,525,410 discloses the use of closely packed with synthetic fibres and a specific zeolite particle that possess antimicrobial activity. In yet another approach, U.S. Pat. Nos. 5,496,860 and 5,561,167 disclose an antibacterial fiber produced through an ion exchange reaction. The antibacterial fiber includes an ion exchange fiber and an antibacterial metal ion entrapped within the ion exchange fiber.
U.S. Pat. No. 5,985,301 discloses a production process of cellulose fiber characterized in that tertiary amine N-oxide is used as a solvent for pulp, and a silver-based antibacterial agent and optionally magnetized mineral ore powder are added, followed by solvent-spinning.
U.S. Pat. No. 6,979,491 describes a method to produce nanosilver based antimicrobial yarn. Silver nitrate solution is reduced using a solution of glucose to produce 1-100 nanometers of silver particles. This solution is then soaked in the solution of nanoparticles.
U.S. Pat. No. 5,454,886 discloses the method of producing nanometallic silver to coat medical devices. The use of physical vapour deposition technique to produce silver nanoparticles render the substrate bactericidal.
A variety of materials have been impregnated with silver to impart beneficially antimicrobial properties, with one example being wound dressings with antimicrobial properties. These dressings may range from simple gauze type dressings to animal derived protein type dressings such as collagen dressings; the composition of the particular dressing depends on the type of wound to be treated. Each of these dressings is used to particular type of wounds depending on their advantages, such as highly economical for simple abrasions and surgical incisions. The chronic wounds are best treated with polymer based dressings. Further polymer based wound dressings use various types of polymeric materials. Generally, they can be classified into two major classes, namely synthetic and naturally derived polymeric materials.
Synthetic materials include polyurethanes, polyvinylpyrolidone (PVP), polyethyleneoxide (PEO) and polyvinyl alcohol (PVA). These materials can be used in combination with other synthetic or natural polymers to achieve specific properties such as moisture retention, re-swelling capability, fluid (exudate) absorption capacity. Similarly, naturally derived polymers or biopolymers, such as collagen and alginates are also exploited for wound healing applications. They are used primarily due to their high water absorption/donating capacity. The biocompatible issue of a material comes to the fore when used for these anti-bacterial dressing applications. Even though they possess these excellent properties, they are usually expensive, and exhibit less exudates absorption and residue deposition on a wound site, thereby limiting their usage. Complimentary to these, hydrocolloid dressings also possess excellent properties that make it viable for wound dressing application. Compared to bacterial cellulose based wound dressing, however, they lack the moisture donating quality. Also, hydrocolloids are known to adhere to the wound bed, causing re-injury upon removal.
As an alternate material, bacterial synthesized cellulose possesses inherent characteristics allowing effective promotion of wound healing. Bacterial cellulose (BC) has certain advantages over plant cellulose, such as, better hydrophilic nature, three dimensional layered structures that allows effective moisture handling capability. Their native dimension and geometry in the fiber form of nanometers (<50 nm) results in high aspect ratio. This has an effect in the water absorption capability per unit area. Their high mechanical strength (78±17 GPa) makes it a unique biopolymer. Bacterial cellulose is highly hydrophilic with a water holding capacity ranging from 60 to 700 times its own weight as is described in U.S. Pat. No. 4,942,128. BC can handle high compressive stress. Ring et al. in U.S. Pat. Nos. 4,588,400, 4,655,758 and 4,788,168 discloses the superior properties of BC which can be modified to produce liquid loaded medical pads. In these studies, BC is produced in a static culture which were loaded with medicaments and liquids. Here, they explain the process of producing the cellulose in a static culture wherein the liquid levels were adjusted by undergoing a series of pressing and soaking to alter the liquid to cellulose ratio.
An artificial skin graft based on microbial cellulose has been disclosed by Farah et al. in their U.S. Pat. No. 4,912,049. This patent describes the method of producing microbial cellulose in a static culture using Acetobacter xylinum, and that is dehydrated while it is stretched. They also suggest the applicability of dehydrated microbial cellulose as an artificial skin substitute with no moisture donation capability and limited exudates absorption capacity.
Instead of producing BC in static cultures, Wan et al. in U.S. Pat. No. 5,846,213 disclosed the method of producing BC in stirred tank bioreactors. They were further dissolved in solvents which were then casted/molded into desired shape and size. The casted cellulose material possesses limited fluid absorption capacity. This also is devoid of the three dimensional structure that is present only in the pellicles produced by a static culture.
Although the above patents recognize the potential use of bacterial cellulose in medical applications, the prior literature has not produced a BC based antibacterial material that is capable of having a moisture management capability with inherent biocompatible nature of the cellulose. Also, an optimum wound healing material requires both liquid absorption/releasing capabilities. The presence of growth factors and anti-microbial material enhances the rate of wound healing.
The present invention describes a procedure to incorporate nano-silver onto bacterial cellulose nanofibres. The antimicrobial properties are also demonstrated. The silver containing fibers can be shaped into any desired form. Alternatively, bacterial cellulose nanofibers can be pre-shaped before silver incorporation.